In electrophotography, multicolor images having resolution and other qualities comparable to those of silver halide photography have been produced in the laboratory. One reason such systems have not been commercially practical is they have generally required liquid developing for high quality. However, recent advances in fine particle dry toners have made low-grain, high-resolution images feasible with dry systems.
U.S. patent application Ser. No. 07/405,258, filed Sep. 11, 1989, entitled TONER FIXING METHOD AND APPARATUS AND IMAGE BEARING RECEIVING SHEET, to Rimai et al, now U.S. Pat. No. 5,089,363 discusses a problem with such high resolution dry images that when they are put through an ordinary roller fuser they both spread, losing resolution, and exhibit a substantial relief image according to the varying thickness of toner layers in the image. The Rimai et al application suggests using a hard ferrotyping belt to embed the toner in a heat softened thermoplastic layer. The combination of relatively high pressure and the heat softened thermoplastic layer both substantially embeds the toner in the layer substantially reducing the relief and also applies a gloss to the image that is highly desirable in such a print.
U.S. patent application Ser. No. 07/409,194, filed Sep. 19, 1989, entitled METHOD AND APPARATUS FOR TREATING TONER IMAGE BEARING RECEIVING SHEETS, Baxter et al, now U.S. Pat. No. 5,112,717 deals with texturizing or adding gloss to a toner image-bearing receiving sheet generally of the type described in the Rimai et al application, that is, having a toner image on a thermoplastic layer. In this application the toner image-bearing sheet is fed between a pair of pressure rollers, one of which may have a texturizing surface and one of which is heated to again soften the thermoplastic layer to help impart the proper gloss or texture to its surface. To prevent offset of the thermoplastic layer onto the pressure roller contacting it, the layer was heated primarily by the roller contacting the side of the receiving sheet opposite the heat softenable layer.
The processes in the above two applications are done without the use of fusing oils because fusing oils leave image defects that are unacceptable with extremely high quality prints.
Japanese Kokai 63-92965 (1988), laid-open Apr. 23, 1988, suggests a method of increasing the gloss of a toner image bearing thermoplastic coated receiving sheet in which the receiving sheet is fed between a pair of pressure rollers to both reduce relief and to add gloss.
European Patent Application 0301585 published Feb. 1, 1989 shows a glazing sheet used to increase the gloss of either a toner image on paper backing or a dye and developer in a thermoplastic coating.
U.S. Pat. No. 4.639.405 shows a post-treatment step to add gloss to a toner image carried on a paper after ordinary fusing. The fixed image bearing paper is dried and then pressed between a pair of heated rollers which increase the gloss of the image.
U.S. Pat. No. 4,780,742 shows a method of increasing the gloss of a fixed toner image by coating it with a thin sheet in the presence of heat and pressure.